Babs they are wearing multi ply briefs which slow you down but you'll also notice that the first part of the video is speed work and then they work up to a max effort single which is slower due to increased weight and band tension.
In comparison to a squat with full gear, that is still fast. Take a look at a meet video of these guys, even with close to 500kgs on their backs it takes a long time to get down to depth.
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